1. Technical Field
The present disclosure relates to differential mode transmission lines, and more particularly to differential mode transmission lines with a weak coupling structure.
2. Related Art
With increasing demands for electronic equipment, portable electronic devices with high transmission speed are developed. However, while these demands are met, many unexpected cases occur concurrently. Taking Printed Circuit Boards (PCBs) for example, with the increasing of the frequency of the signal transmission, electromagnetic waves are likely to be emitted outwards, resulting in the so-called electromagnetic interference (EMI). The EMI is not only harmful to the human body, but also may makes the PCBs fail to meet the required safety standards.
Accordingly, because differential mode transmission lines are resistant to the EMI and not easily be affected by grounding voltage disturbances, the differential mode transmission lines are widely used. However, in order to layout a complex circuit in a limited PCB space, the differential mode transmission lines must have many corners. Due to those corners, the lengths of the differential mode transmission lines are unequal, resulting common mode noises. Descriptions are given below with reference to the drawings.
FIG. 1 is a schematic view of a conventional differential mode transmission line. The conventional differential mode transmission lines include a first transmission line 100 and a second transmission line 200. The first transmission line 100 includes a first wiring portion 120, a second wiring portion 140, and a third wiring portion 160. The second transmission line 200 includes a first wiring portion 220, a second wiring portion 240, and a third wiring portion 260. Furthermore, the first transmission line and the second transmission line define a first pitch D. That is, distance between the first transmission line and the second transmission line remains the same.
In the structure shown in FIG. 1, a corner exists between the first wiring portion 120 and the second wiring portion 140 of the first transmission line 100. Similarly, a corner exists between the second wiring portion 140 and the third wiring portion 160. The second transmission line 200 has similar structures. Due to those corners, the lengths of the transmission lines are unequal to each other. It can be seen from FIG. 1, since those turns of the conventional differential mode transmission lines cause difference in length due to the geometrical structure, common mode noises are generated while signals are transmitted in those transmission lines. Furthermore, at an uncompensated segment (that is, the second wiring portion of the preceding transmission line), because the transmission rates of the common mode noises are different from those of the signals, the common mode noises generated at the corners cannot be eliminated at the next turning site, thereby resulting in the EMI, which may make the product fail to meet the safety standards.